Many forms of digital communications used today require separate wires to carry the digital messages from one place to another. The only exception to this trend is RF (Radio Frequency) based systems. Systems employing RF technology transmit a radio signal through the air in the same manner as a cellular phone.
When a non-RF system uses dedicated wires for communications, those wires are often referred to collectively as the “communications bus.” A communications bus refers to a group of wires whose responsibility it is to get information from the source to the intended receiver or from the receiver to the source. The reason that is done is to denote the fact that those wires are acting as a stand-alone sub-system and must be isolated from other sub-systems. In more complex systems it is possible to have several communications buses. This would become necessary if the different communications sub-systems were incompatible and the digital information being conveyed by one sub-system would interfere with the operation of another sub-system.
FIG. 1 is an illustration of a simple prior art communications bus system that might be encountered in a mobile DC (Direct Current) application. This type of system is referred to as using conducted signals or emissions. This means the signal itself is following a conductive path, e.g., a conductive wire of the communications bus. This results in a more controlled message path, a message that can work at low power levels and reduced chances of the signal interfering with other electronic devices.
Conducted emission systems to date have had one distinct disadvantage. A conductive path can also carry many types of noise. This noise can come from many sources. Examples would be a motor, solenoid, generator or certain types of lights. Radiated emissions of sufficient power can also attach themselves to a conductive path.
One prior art type of communications method that is gaining in popularity is the Radio Frequency (RF) system, as shown in FIG. 2. As explained previously, this works in much the same manner as a cellular phone.
RF systems use what is referred to as a radiated transmission or emission. That type of signal is intentionally transmitted into the air surrounding an antenna in all directions. Radiated emission devices are tightly controlled by the FCC (Federal Communications Commission) due to their tendency to interfere with other electronic devices. Conventional RF devices must transmit within well-defined ranges of frequencies that have been established by the FCC. Most of the FCC frequencies require a license to operate a transmitting device. There are specific frequencies set aside for low power devices such as the ones used in the RF devices currently being discussed. These devices collectively are sometimes referred to as “Part 15” devices as that is the section of the FCC rules they operate within.
To initiate a RF system, an input device receives information from a pre-determined source. This could vary from a sensor monitoring a certain parameter to a switch sending a “go” or “stop” command. Once the incoming information is recognized, the input controller generates a message for transmission. The input controller would then broadcast that coded message through the air to all other controllers in the system. One disadvantage to this scheme is that any electronic device capable of receiving air-born messages and in close proximity to the transmitting device would also receive the message.
In order to minimize the possibility of an unintended device responding to an incoming message the originating controller would encode the message with a special recognition number. To further reduce the possibility of miscommunication, some systems employ what is referred to as a Spread Spectrum Frequency Hopping communications scheme. What that means is that instead of broadcasting on only one frequency, the controller sends out portions of its message on different frequencies. The receiving device must be directed to know what frequency to start at and what other frequencies to hop to as the incoming message progresses.
The need remains for simplified digital communication systems and methods that do not require dedicated communications wires and that send digital messages over the same wires that are used to power DC systems.